Eliminate complier warning and remove superfluous call to empty()
Eliminate complier warning and remove superfluous call to empty()

Appears when complied with -O3 optimization level

Compile: UARTSerial.cpp
../drivers/UARTSerial.cpp: In member function 'void mbed::UARTSerial::tx_irq()':
../drivers/UARTSerial.cpp:314:31: warning: 'data' may be used uninitialized in this function [-Wmaybe-uninitialized]
         SerialBase::_base_putc(data);

betzw/mbed-os,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,betzw/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os